Anaerobic adhesive and sealant compositions have come into prominence in recent years because of their superior performance and convenience as thread-lockers and sealants and for various other important uses in machine assembly. Anaerobic compositions have been described in detail in U.S. Pat. Nos. 2,895,950; 3,218,305; 3,425,988; and 3,435,012, among others, as well as British Specification No. 1,412,940. The compositions depend on the generation of free radicals, normally by a redox-type reaction catalyzed by the metallic surfaces to which the compositions are applied in use, or by pre-applied primers. It has also been found that some of these compositions can be cured by exposing them to non-ionizing radiation, such as UV light; this is disclosed in French Pat. No. 73.17729.
An anaerobic composition has as required elements a free radical polymerizable monomer (normally a mono-, di- or triacrylate or methacrylate) and a peroxy initiator or a photo-initiator, or both. It may include accelerators, such as amines and organic sulfimides. When such a composition is required to cure under irradiation such as UV, it must also include at least one photo-initiator or sensitizer such as benzoin methyl ether or one of the initiators described in Australian Patent 469,564. The composition may also include polymeric thickeners, coloring agents including those pigments used in reflective coatings, and plasticizers. Free radical stabilizers may also be required to balance the shelf life of the compositions against the speed of cure. These stabilizers are normally of the quinone type, and the preferred quinones are beta-naphthoquinone, 2-methoxy-1,4-naphthoquinone, and p-benzoquinone. Other stabilizers are disclosed in U.S. Pat. No. 3,043,820. Compatible plasticizers may optionally be used, the preferred plasticizers being selected from the group of polyester polymeric plasticizers described in U.S. Pat. No. 3,794,610.
Among the most common and useful monomers are those commonly designated polyethyleneglycol dimethacrylate (PEGMA), tetra-ethyleneglycol dimethacrylate (TEGMA) and triethyleneglycol dimethacrylate (TRIEGMA). These and other anaerobic monomers are typically manufactured by processes using the well-known catalytic action of strong acids such as sulfuric acid, methane sulfonic acid and para-toluene sulfonic acid. The monomers as commercially available usually contain significant residual concentrations of such strong acids as impurities. In addition, the manufacturing process typically forms precursors of strong acids, e.g., labile esters, which also remain in the monomers as sold. These residual precursors tend to hydrolyze or decompose during storage of the monomers, even at room temperature, to form strong acids in situ.
It has been known for some years that the presence of such strong acids and precursors tends to cause an undesirably low initial pH as well as a gradual reduction of pH of the monomer (known as "acid drift") which, in turn, degrade the shelf life and performance of the anaerobic composition. Various methods have been tried to correct and control acid drift. For example, alumina and inorganic bases such as sodium hydroxide have been used. Such chemicals will neutralize acids and are more or less effective stoichiometrically; however, they can not effectively be used to control the in situ acid formation since the necessary excess of such bases would itself destabilize the anaerobic composition.
Similarly, organic bases such as amines have been tried. While they will also react stoichiometrically, they cannot be left present in excess since they tend to accelerate the anaerobic cure reaction and thus destabilize the composition.
Accordingly, a method of correcting too low pH and controlling acid drift in monomer would be a very worthwhile advance in anaerobic technology.
While epoxy compounds have been used to stabilize various polymerized systems, e.g., polyvinyl chloride resin (see, for example, May & Tanaka, Eds, "Epoxy Resins," Chapter 10, Marcel Dekker, N.Y. (1973)), they have not been kown for use in reactive, "living" systems such as anaerobic formulations.